This invention relates to methods and apparatus for the control of power converters and the like circuitry, and particular to methods and apparatus for such control that do not require direct sensing of input and/or output voltages. The invention relates in particular to methods and apparatus for determining the operating characteristics of a discharge lamp (for example a fluorescent discharge lamp or a high-intensity discharge lamp) such as lamp power, lamp resistance and input voltage.
Switching power converters are widely used in a large number of domestic and industrial applications. Examples include computer systems, motor drivers, and uninterruptible power supplies. With recent advances in semiconductor technologies and electronic packaging techniques, much research has been done on new power circuit topologies, switching scheme, and control techniques for improving the converter efficiency, electrical specifications, and power densityxe2x80x94all the time meeting various industrial standards. Examples of well known power converter topologies include buck converters, boost converters, buck boost converters, flyback converters and forward converters.
An underlying concept of power electronics is to be able to use low-level signals to control high power converter outputs. Conventionally this requires a comparison of the actual output voltage with a desired reference voltage and then giving commands to the power converters. However, it is common practice that the power conversion stage and the control circuit be isolated in order to avoid noise coupling and grounding problems. In some situations input and output isolation in the power conversion stage is also desirable or necessary. These isolation requirements means that signal-power interface techniques such as transformer coupling and optical isolation are necessary to achieve output regulation. These requirements substantially increase the cost and complexity of power converters.
One solution to this difficulty is to control switching power converters by using current sensors only and without requiring the use of voltage sensors. Such a system was described in T. Ohnuki, O. Miyashita, P. Lataire and G. Matson IEEE Transactions on Power Electronics, Vol. 14 No. 2 March 1999. In the system proposed in this paper only current sensors are used that generate signals in response to the currents flowing in inductors. The sensed current can in theory be used to obtain the input and output voltages so as to provide control information. The use of current sensors alone has a number of advantages including a reduction in the number of sensors needed, and it obviates the need to use a dissipative voltage divider, such as a resistive network, to obtain the input voltage in feedforward arrangements and in output voltage regulation. Additionally, no voltage isolator (such as an optical coupler) is needed to isolate the high-voltage output and the low-voltage control signals. This has an additional advantage in that some optical isolators have a finite linear range, eliminating the need for such isolators therefore increases the practical voltage measurement range.
Since the current can be sensed using a contactless flux linkage sensor such as a Hall effect sensor, electrical isolation between the power conversion stage and the control state may be achieved easily. This has the effect that the power and ground signals can be separated inherently so as to reduce noise coupling.
The proposal of this piece of prior art has, however, a number of practical drawbacks. Most importantly, it assumes that the circuit is an ideal circuit, which in reality no such circuit ever would be. In addition, the current rather than being continuously sensed is simply sampled once in every switching cycle with the inevitable approximations and inaccuracies that this implies. An improvement on this technique is disclosed in the Applicants application Ser. No. 09/524,041, U.S. Pat. No. 6,297,621, which is incorporated herein by reference.
According to the present invention there is provided apparatus for determining the characteristic(s) of a discharge lamp operated by a high-frequency electronic ballast including a resonant tank comprising an inductor and a capacitor, wherein said characteristic(s) are selected from the group consisting of lamp input voltage, lamp resistance and lamp power, comprising means for measuring the inductor voltage and/or current, and means for determining said characteristic(s) from the measured inductor voltage and/or current.
By means of this invention, at least in its preferred forms, the control variables (such as input and output voltages) and the characteristics of the load (such as a discharge lamp) may be obtained more easily than in the prior art. For example, any significant changes in the lamp resistance may indicate lamp instability or failure, and since by means of the present invention the lamp resistance can be monitored remotely through measurements of the inductor voltage or current, the lamp resistance can be monitored remotely to identify any instability or failure. In addition, the present invention may be employed to detect load condition to facilitate load monitoring (for stability control for example).
In preferred embodiments of the invention the apparatus may comprise means for obtaining the lamp current and the maximum inductor voltage from the measured inductor voltage and/or current, and the characteristic(s) determining means may then determine the characteristics from the lamp current and maximum inductor voltage.
In one embodiment the apparatus comprises a sensor for measuring the inductor voltage, means for differentiating the inductor voltage to obtain a capacitor current, means for integrating the inductor voltage to obtain an inductor current, means for obtaining the lamp current from the capacitor current and the inductor current, and peak detector means for detecting the maximum inductor voltage.
In another embodiment the apparatus comprises a sensor for measuring inductor current, means for differentiating the inductor current to obtain an inductor voltage, means for differentiating the inductor voltage to obtain an capacitor current, means for obtaining the lamp current from the capacitor current and the inductor current, and peak detector means for detecting the maximum inductor voltage.
Viewed from another aspect the present invention provides apparatus for determining the lamp power, lamp resistance and input voltage of a discharge lamp driven by a power converter circuit including an inductor, comprising means for measuring the inductor voltage or inductor current, and means for determining the lamp power, lamp resistance and/or input voltage from said measured inductor voltage or inductor current.
Viewed from a still further aspect the invention provides a method for determining the characteristic(s) of a discharge lamp operated by a high-frequency electronic ballast including a resonant tank comprising an inductor and a capacitor, wherein said characteristic(s) are selected from the group consisting of lamp input voltage, lamp resistance and lamp power, comprising measuring the inductor voltage and/or current, and determining said characteristic(s) from the measured inductor voltage and/or current.
More generally still the present invention provides apparatus for determining the input and/or output voltages of a power electronics converter circuit including an inductor, comprising means for sensing the voltage across the inductor, and means for calculating the input and output voltages from the inductor voltage. Preferably the apparatus may also be able to calculate the load resistance from the inductor voltage.